how long does it take to drain?

just wondering if anyone knows how long on average it takes to drain one boob? cos i'm feeding jessica until she starts to get fussy on the one boob and then i put her on the other and she's fine. but i'm not sure if i've been putting her on the second boob too early. and does it really matter if i do?
she's 15 days old now and bfing seems to be going fine, was just a bit worried bout this.
any help is appreciated! xxx